Rotational molding, more commonly known as rotomolding, is widely used for molding hollow articles such as outdoor sports equipment, storage tanks and refuse containers. To rotomold a part, a powdered resin is charged inside a mold shell, which is then rotated on two axes and heated inside an oven to cause the resin to adhere to the inside of the mold. After sufficient heating time, the mold is moved to a cooling chamber, and after cooling, the molded part is removed to begin another molding cycle. A more detailed discussion of rotomolding may be found in Modern Plastics Encyclopedia 1990, pages 317-318.
Rotational molding primarily uses polyolefin resins, with thermoplastic polymers of ethylene being principally used. Polyethylene resin suitable for rotational molding is generally a medium density resin.
Foaming agents are useful to reduce part weight, provide a higher insulation value, increase stiffness and reduce resin cost. Generally, foaming agents may be classified as "physical" or "chemical" foaming agents. Typically, physical foaming agents are normally gaseous at the temperature at which the plastic mass is to be foamed. Chemical foaming agents are typically compositions which decompose or react to form a gas. Heat decomposable, foaming agents include organic compounds such as azodicarbonamide, 4,4'-oxybisbenzene-sulfonyl hydrazide, and p-toluenesulfonyl hydrazide; disadvantages include cost and unpleasant odor. Carbon dioxide-releasing, foaming agents include inorganic salts such as sodium, ammonium, and potassium bicarbonate. A foaming coagent is advantageously used with a reactive type, chemical foaming agent, and for an acid-released, foaming agent, a fatty acid such as stearic acid or a mild organic acid such as citric acid is often used as the coagent.
Useful additives when foaming thermoplastics include nucleating agents. A nucleating agent promotes uniform cell size and reduces the existence of surges and voids in the foam. As exemplified by U.S. Pat. Nos. 4,107,354, 4,110,269 and 4,275,168, nucleating agents useful when foaming polyolefins include calcium carbonate; pigments such as carbon black; and silicates such as kaolins, talc, aluminum silicate, magnesium silicate, and calcium silicate.
Despite advances in polyolefin-based, rotomolding compositions, there is a need for an improved polyolefin-based composition and method for providing rotomolded articles having an inside layer or core of one resin, and an exterior layer or skin of a different resin. It would be beneficial if the inside layer or core could be selectively foamed or selectively strengthened, and if the exterior layer or skin could be provided with improved toughness.